1. Field of the Invention
The present invention concerns the control of transistors and more particularly MOS transistors (MOSFETs).
2. Description of the Related Art
In certain applications, an MOS transistor is included in an electrical conversion bridge, such as a voltage rectifying bridge. Such an electrical conversion bridge, for example an inverter, can be used reversibly. Thus it is known how to make function as an alternator an electric motor controlled by a reversible inverter comprising MOSFETs, using an internal diode of the MOSFETs as a rectifying element. This amounts to using the inverter as a diode bridge. However, the voltage drop at the terminals of the internal diodes gives rise to significant electrical losses.
This is why each MOS transistor is switched into the on state when a reverse current (negative current) flows between its drain and its source (phase I), and into the off state the rest of the time (phase II).
In addition, in order to increase the intensity of the current allowable in each switching unit, it is known from the U.S. Pat. No. 6,430,071 how to employ switching assemblies comprising a plurality of transistors connected in parallel. These switching assemblies used in electrical conversion bridges are controlled alternately in off saturated mode and in on saturated mode. Nevertheless, because of the voltage drop at the terminals of the internal diodes, significant switching losses are observed.
It is also known, from the document WO-A-2004/034439 how to control the MOSFETs in linear mode in the on state, this is to say to control their internal resistance linearly in order to keep constant the voltage at the drain/source terminals of the transistor. This is achieved by means of a control.
However, in the case of currents of high amplitude, the control imposes an instruction on the transistor such that the latter can go into closed-switch mode, the internal resistance of the transistor then reaching its minimum value (ON state). When the transistor must be then switched into the non-conducting state (OFF state), the command must make it pass from the ON state to the OFF state. For this purpose, it is necessary to discharge the electrical charges accumulated in the transistor. Consequently there is in general observed a delay in the switching that gives rise to the appearance of a high overlap current. This may reduce the performance of the conversion bridge, in particular the radio interference, losses by Joule effect and stray ripples on the on board system.